Interpretive Summary: Increasing global atmospheric CO2 concentration has led to concerns regarding its potential effects on grasslands and the long-term storage of C and N in soil. This study examined responses to elevated CO2 in a grass ecosystem invaded with a leguminous shrub. Seedlings of Acacia along with grass species were grown for 13 months at three different levels of atmospheric CO2 concentrations. Elevated CO2 increased both C and N inputs from plant growth which would result in higher soil C from litter fall, root turnover and excretions. Results from the incubation indicated an initial (20 days) decrease in N mineralization which resulted in no change in C mineralization. However, after 40 days, an increase in both C and N mineralization was observed. These increases would indicate that increases in soil C storage may not occur in grass ecosystems that are invaded with Acacia over the long-term.

Technical Abstract:
Increasing global atmospheric CO2 concentration has led to concerns regarding its potential effects on terrestrial ecosystem and the long-term storage of C and N in soil. This study examined responses to elevated CO2 in a grass ecosystem invaded with a leguminous shrub Acacia farnesiana (L.) Willd (huisache). Seedlings of Acacia along with grass species were grown for 13 months at CO2 concentrations of 385 (ambient), 690, and 980 µmol mol-1. Elevated CO2 increased both C and N inputs from plant growth which would result in higher soil C from litter fall, root turnover and excretions. Results from the incubation indicated an initial (20 days) decrease in N mineralization which resulted in no change in C mineralization. However, after 40 days, an increase in both C and N mineralization was observed. These increases would indicate that increases in soil C storage may not occur in grass ecosystems that are invaded with Acacia over the long-term.